1. Field of the Invention
The present invention relates to a transceiver processing user signals in a telecommunication system, and, in particular, to compensation for different transmission rates of digital signals in transmit and receive paths that are generated between the transceiver and a user.
2. Description of the Related Art
Transceivers commonly process signals transferred between a user and a telecommunications network. One common form of signal processing is coordination of data transfer between a user and the communication network for a particular form of modem transmission. For such coordination, the modem must generate and receive combinations of tones that represent either training, connection supervision, or modulated/coded data as specified by the particular standard, such as V.90 or V.34. For example, a digital multi-tone (DMT) transceiver may be included in a modem to generate and receive such combinations of tones. Modem signals are transferred between the user and the DMT transceiver as analog signals converted to the digital domain by a coder/decoder (codec), such as a pulse code modulation (PCM) codec. The codec samples the analog modem signal received from the user and/or generates an analog modem signal for the user from digital samples generated from the output of the DMT transceiver. Since the tones employed are of finite duration and combinations of frequency and phase of distinct carriers, the DMT transceiver operates in the frequency domain on complex coefficients representing the signals. The Fourier transform, well known in the art, may be employed to convert between the sampled modem signals and the complex coefficients that are processed by the DMT transceiver.
FIG. 1 shows a prior art system 100 processing modem signals between a user and a telecommunications network. System 100 includes DMT transceiver 101 processing the modem signals in transmit (user to network) and receive (network to user) paths and codec 102 providing conversion between the bi-directional analog signals and the digital signals in the transmit and receive paths. Codec 102 samples analog modem signals from the user to provide a sequence of digital samples in the transmit path, and codec 102 constructs analog modem signals for the user from a sequence of digital samples in the receive path. In the receive path, system 100 includes time domain equalizer (TEQ) 106 and fast Fourier transform (FFT) processor 107. TEQ 106 equalizes the analog waveform represented by the digital samples based on the type of FFT transform subsequently applied to the sampled modem signal by FFT processor 107. Filtering may equalize the analog waveform to shorten channel length effects, such as memory or signal dispersion. FFT processor 107 applies a 256-point complex FFT to a block of 512 digital samples (representing the equalized, sampled analog signal) to generate 256 complex coefficient values (i.e., 256 real and 256 imaginary coefficients) for processing by the DMT transceiver 101.
In the transmit path, system 100 includes inverse FFT (IFFT) processor 103, copy and add module (CP ADD) 104, and upsampler 105. IFFT processor 103 buffers 32 complex coefficients representing (in the FFT transform frequency domain) the modem tone combinations generated by the DMT transceiver 101. IFFT processor 103 then applies a 32-point complex IFFT transform to the 32 complex coefficients to generate a block of 64 digital samples that represent a sampled analog modem signal. CP ADD 104 copies the first four digital samples in the sequence and appends them to the end of the sequence (after sample number 64). As is known the art, copying digital samples from the beginning of the block and appending them to the end of the block ensures that the signal represented by the digital samples generated by the IFFT processor 103 is periodic.
DMT transceiver 101 may generate digital samples in the receive path from complex-valued coefficients in the frequency domain at a so-called “DMT transmission rate” that is different from the rate of digital samples generated by codec 102 in the receive path. The DMT transmission rate in the transmit path without any rate compensation is generally less than the transmission rate in the receive path. However, codec 102 desirably employs analog-to-digital (A/D) converters (and possibly pulse code modulation (PCM) encoders) in the receive path operating at the same rate as its digital-to-analog (D/A) converters (and possibly PCM decoders) in the transmit path.
Consequently, the difference in transmission rate of the digital samples between the transmit and receive paths is compensated for in the receive path by interpolation of the digital samples provided from the CP ADD 104. Interpolation of the prior art systems such as shown in FIG. 1 is typically performed by an interpolating filter or upsampler, such as upsampler 105. Upsampler 105 interpolates by 8 the block of 68 samples from CP ADD 104 and then may select a subset of the interpolated samples (e.g., by truncation) to provide a block of 512 digital samples. Such compensation may require considerable processing by, and hence considerable circuit area and power in, an integrated circuit implementation.